No, this is complete rubbish. The idea that CO2 concentration increases of 1% per year (i.e. about 3.8 ppm per year) represent “business as usual” is nothing but a bald-faced lie.

CO2 concentrations have been increasing by about HALF that value over the last 20 years:

http://www.cmdl.noaa.gov/gallery/ccgg_figures/co2trend_global

No, this is incorrect for many reasons:

1) It is not just CO2 that has contributed to the “discernible climate shift from preindustrial background.” It’s true that CO2 has gone from about 280 ppm before the industrial revolution to approximately 380 ppm today (i.e., has increased by approximately 100 ppm).

But the atmospheric methane concentration has also more than doubled, and nitrous oxide (N20) levels have increased. Also, chlorofluorocarbons are present that weren’t present at all in the “preindustrial background.” Tropospheric ozone (aka, Los Angeles type “smog”) is also higher than in the preindustrial background.

All these effects add to the forcing from carbon dioxide:

http://intl.pnas.org/cgi/content/full/95/22/12753/F2

If we take into effect *all* the greenhouse gases (and neglect aerosols…some of which are thought to cool, and some of which are thought to warm), the current “CO2 equivalent” concentration is more like 60% above the pre-industrial value, or more like 450 ppm.

2) More importantly, absent feedbacks, the world temperature does NOT respond linearly changes in CO2 concentration, i.e. each 80 ppm increase doesn’t produce an equal change in temperature. Absent feedbacks, the world temperature responds changes linearly with LOGARITHMIC changes in CO2 concentrations. From the IPCC First Assessment Report, the following equation was given for radiative forcing versus CO2 concentration:

Delta F = 6.3*ln(C/Co), where

Delta F = Change in radiative forcing in Watts/square meter,

C = Carbon dioxide concentration at the time of interest

Co = Concentration at the previous time of interest.

Plugging in values of 280 ppm and 360 ppm, we get:

delta F = 6.3 ln(360/280) = 1.58 Watts/m2

But going up 80 more ppm, from 360 to 440 ppm, produces:

delta F = 6.3 ln(440/360) = 1.22 Watts/m2.

In other words, each 80 ppm increment produces a SMALLER temperature increase (absent feedbacks).

“When your two scenarios diverge by about that much, amounting to atmospheric concentration differences of about 80 ppmv you are entering statistically different climates.”

No, that is not correct, for the reasons I gave above.

Anyway, if I understand the question correctly, it’s easy to estimate. It took a change of about 80 ppmv CO2 to create a discernible climate shift from the preindustrial background. When your two scenarios diverge by about that much, amounting to atmospheric concentration differences of about 80 ppmv you are entering statistically different climates. That is the edge case. Comparing to the business-as-usual increases of about 1% per year, we are looking at twenty years of so.

More realistic strategies (opposed to abruptly ending all anthropogenic emissions) will take even longer to have a discernible effect even compared to a hypothetical alternative realization, one which will not be perceived by the public.

Ouch. Thanks for the exercise. That is really a very awkward result, isn’t it?

OK, the scenario is that, in the year 2020, CO2 emissions go from whatever they are at that time (my guess is about 30 percent above 1990 values) to absolutely zero. And stay that way forever. The CO2 atmospheric concentration in the year 2020 should be approximately 405 ppm.

The question is, “How long would it take for concentrations to drop to 300 ppm (from 405 ppm)?”

I’m too lazy to do a careful analysis, but currently something like 50% of the CO2 emitted by humans doesn’t show up as concentration increases in the atmosphere. And concentration in the atmosphere is increasing by about 1.8 ppm per year. So if emissions dropped to zero, concentration should *fall* by about 1.8 ppm per year. (Again, I’m too lazy to look up actual values.)

Since we’re talking about 405 ppm – 300 ppm = 105 ppm…dividing that by 1.8 ppm per year equals about 60 years. But actually, the decline would not be that steep…the decline per year should get smaller and smaller as one got closer to 300 ppm. (In fact, if you’d chosen a value of 280 ppm, which was the value before the Industrial revolution started, the answer could theoretically be “an infinite amount of time…or at least until the start of a new Ice Age.)

So my guesstimate would be not less than 60 years…nor more than a few hundred. (If you want a better estimate, you’ll have to send some money. )

If instead of halting anthropogenic CO2 emissions instantaneously on 31 December 2005 we halted them as fast as we could replace our cars, buildings and factories with radically more efficient models; and simultaneously used wind turbines, building-integrated solar, and biomass to supply our greatly reduced energy demand, much more than climate would change:

The air would be pristine. We would all be healthier. We would be much more comfortable in our quieter cars and well-insulated homes. Owning these vastly superior assets would make us wealthier. Energy security would be available to every country, and no longer be a casus belli. We would no longer worry about sending petrodollars to the Middle East or about nuclear power plant safety and security. With all these advantages, even China wouldn’t mind having wasted $18 billion on Chevron.

And Amory Lovins (cited by commenters Murray Duffin and Lynn Vincentnathan on other threads) would be happy indeed that we finally listened.

1a) “What is the probability that the Earth will be moving into an Ice Age by 2100? Is it a 1 in 1000 or 1 in 10 chance?”

My opinion is the probablity is less than 1 in 10, but more than 1 in 1000. Just to take a very crude guess, I’d guess 1 in 100.

1b) “If after all our effort to stabilise temperature the earth starts to dramatically cool what should we do, try and make it warmer?”

In my opinion, a cooling of 2-3 degrees Celsius from the current temperature would be much, much worse than a warming of 2-3 degrees Celsius. So, yes, in my opinion, if the earth appears likely to dramatically cool, we definitely should try to stop that cooling.

2) “This leads to my second question. How convinced are we that any action we take will have a Net positive benefit for mankind?”

This is an extremely important and excellent question. I discuss this very question on my global warming website (caution: should be viewed with a pop-up blocker):

http://markbahner.50g.com/

http://markbahner.50g.com/is_it_bad.htm

To my knowledge, there has NEVER been a careful and unbiased study that asks the question: “Is there an ‘optimum’ temperature for the earth, and if so, what is it?”

Even though I haven’t formally studied the matter, my initial guesstimate going into the study would be:

1) Yes, there is an “optimum” temperature for the earth, and

2) it is somewhere between 1 degree Celsius cooler than at present and 2 degrees Celsius warmer than at present.

Any such study would involve a very significant number of “value” judgements…e.g. “Is it better to have good skiing conditions, or more pleasant days at the beach?”

P.S. I think one way that one could somewhat evaluate what the “optimum” temperature is, would be to look at movements of people within countries, and from country to country. For example, are people in the United States moving to the South or the North? What about people in Canada? Britain? And so on.

Here are my guesses for the “50% probability values” for warming from 1990 to 2100, based on those three scenarios.

a) Halting (bringing emissions instantaneously to zero) in 2020: Approximately 0.8 degrees Celsius of warming from 1990 to 2100.

b) Cutting CO2 emissions to 1990 levels by 2012 (note: I’m assuming that they are then kept at 1990 levels for the rest of the century): Approximately 1.0 degree Celsius of warming from 1990 to 2100.

c) Cutting global emissions to 50% (of current levels…presumably in a straight-line reduction) by 2030 (and then presumably keeping them at that level for the rest of the century): Approximately 1.0 degree Celsius of warming from 1990 to 2100.

In other words, in my opinion three scenarios are almost indistinguishable from one another, in terms of the expected temperature rise from 1990 to 2100. (This is mainly because of the rise of approximately 0.2 degrees Celsius that has already occurred from 1990 to 2005, plus an assumed 0.5 degrees Celsius warming that is “in the pipeline”…i.e., which will happen even if CO2 emissions were instantaneously stopped.)

P.S. Since I didn’t express the first scenario in these terms, my estimate of the “50 percent probability” value for warming from 1990 to 2100 that would result from an instantaneous cut to zero emissions right in 2005 (and maintaining at zero) would be about 0.5 degrees Celsius. In other words, the slightly *falling* CO2 levels (resulting from zero emissions) would actually shave off the warming that has occurred from 1990 to 2005, and the only warming would be the 0.5 degrees that’s already in the oceans.

So for all four scenarios, the expected warming is somewhere between 0.5 degree Celsius (for cutting emissions to zero immediately) and 1.0 degree Celsius (for cutting emissions to 1990 levels by 2012, or for cutting emissions by 50% by 2030).

P.P.S. Note that my “50 percent probability estimate” for the temperature increase that would result from absolutely no deviation from “business as usual” is 1.2 degrees Celsius. This corresponds to industrial emissions of CO2 peaking at approximately 1.5 times their 1990 levels in 2030, and declining to approximately 0.7 times their 1990 levels by 2100.

http://markbahner.typepad.com/random_thoughts/2005/01/prujections_ipc.html

All rough estimates, E&OE.

Of course all these also depend to some extent on understanding and believing that model outputs are useful – there would no doubt be sceptics who would claim it was all just natural variability, whatever happened. Also, note that being undetectable (at a certain confidence level) doesn’t mean that an effect does not exist.

http://sciencepolicy.colorado.edu/prometheus/archives/climate_change/000137update_on_european_g.html

Experiences with respect to actual efforts to reduce emissions is precisely why we should be having a wide-ranging discussion of policy options.